In next generation wireless communication systems, multimedia data can be transmitted with high quality at a high speed under limited radio resources. To achieve this, spectral efficiency needs to be maximized since a radio channel has a limited bandwidth. In addition, inter-symbol interference and frequency selective fading, which occur during high-speed transmission, need to be overcome.
In order to improve performance of the wireless communication system, a closed-loop transmission scheme using channel condition between a base station (BS) and a user equipment (UE) has been introduced. An adaptive modulation and coding (AMC) scheme improves link performance by adjusting modulation and coding scheme (MCS) by using feedback of channel condition information.
In general, the UE informs the BS of downlink channel condition in a well-known format, e.g., a channel quality indicator (CQI). The BS can receive the downlink channel condition from all UEs and perform frequency selective scheduling. To perform the frequency selective scheduling in uplink, the BS has to know uplink channel condition.
A reference signal is used to estimate the channel condition. The reference signal is previously known to both the BS and the UE, and is also referred to as a pilot. An uplink reference signal has two types of signals, i.e., a demodulation reference signal and a sounding reference signal. The demodulation reference signal is used to estimate a channel for data demodulation. The sounding reference signal is used in user scheduling irrespective of data transmission.
A variety of uplink control signal is transmitted on uplink control channel. Examples of the uplink control signal are an acknowledgment (ACK)/not-acknowledgement (NACK) signal used to perform hybrid automatic repeat request (HARQ), a channel quality indicator (CQI) indicating downlink channel quality, a precoding matrix index (PMI), a rank indicator (RI), etc.
Uplink transmission is performed by the UE. Thus, it is important for the UE to have low peak-to-average power ratio (PAPR) in order to decrease battery consumption. For this, a modulation scheme having a single carrier characteristic can be used in uplink transmission. The sounding reference signal is not related to uplink control signal. Therefore, when the sounding reference signal is transmitted on the uplink control channel, it is difficult to preserve the single carrier characteristic. In addition, if the uplink control signal and the sounding reference signal are separately transmitted, it is difficult to improve spectral efficiency.